1. Field of the Invention
The present invention relates to a method of strengthening articles formed from a low carbon, low alloy deep drawing cold rolled and annealed strip and sheet stock which is capable of responding to the strengthening method. In particular, a preferred embodiment of this invention involves the concept of producing stamped or deep drawn parts from a highly formable, low strength, deep drawing-quality steel, and subsequently strengthening the parts after forming by alloy-nitrogen precipitation strengthening. Thus, it is possible to produce high strength, complex stampings which require a highly formable steel to make, which cannot be made from high strength sheet steels because of their low degree of formability relative to deep drawing-quality sheet steels.
2. Description of the Prior Art
U.S. Pat. No. 3,847,682, issued Nov. 12, 1974 to Rollin E. Hook, discloses a method of increasing the yield strength of a low carbon steel sheet stock or an article formed therefrom by heating in an atmosphere comprising ammonia and hydrogen. A deoxidized, low carbon steel containing from about 0.002 to about 0.015% carbon, up to about 0.012% nitrogen, up to about 0.08% aluminum, a nitride-forming element chosen from the group consisting of titanium, columbium, zirconium, and mixtures thereof, in amounts such that titanium in solution is from about 0.02 to about 0.2%, columbium in solution is from about 0.025 to about 0.3%, and zirconium in solution is from about 0.025 to about 0.3%, and balance essentially iron, is heat treated at 1100.degree. to 1350.degree. F. in an atmosphere containing ammonia in an amount insufficient, at the temperature and time involved, to permit formation of iron nitride. Within a preferred temperature range of 1100.degree. to 1300.degree. F. the preferred furnace atmosphere comprises ammonia-hydrogen mixtures having 3 to 6% by volume ammonia. The maximum ammonia concentrations which can be used within this temperature range and which avoid the formation of an iron nitride surface layer are as follows:
______________________________________ 1100.degree. F about 10% ammonia 1200.degree. F about 6% ammonia 1300.degree. F about 3% ammonia ______________________________________
This patent further discloses that nitrogen taken into solid solution as a result of the alloy-nitrogen precipitation strengthening step can present weldability problems and can result in high ductile-to-brittle Charpy impact transition temperatures. However, if the nitriding step is followed by a denitriding step which involves annealing in pure hydrogen at about 1200.degree. F. for at least 2 hours, the excess nitrogen is removed (with only a 10-20% reduction in yield strength), thereby eliminating welding porosity and substantially reducing the ductile-to-brittle transition temperature with consequent improvement in Charpy impact energy values.
Case hardening by heat treating in an ammonia-containing atmosphere to form an iron-nitrogen austenitic structure which is transformed by quenching to a martensitic structure having high surface hardness, has been practiced for many years, and a typical process relating to nitriding of a "Nitralloy" type steel is disclosed in U.S. Pat. No. 3,399,085, issued Aug. 27, 1968 to H. E. Knechtel et al.
U.S. Pat. No. 3,215,567, issued Nov. 2, 1965 to H. Yoshida; U.S. Pat. No. 3,281,286, issued Oct. 25, 1966 to M. Shimizu et al; and U.S. Pat. No. 3,303,060, issued Feb. 7, 1967 to M. Shimizu et al., disclose denitriding and decarburizing of cold-rolled low carbon steel sheet and strip stock. When denitriding is desired in these prior art processes, an atmosphere containing in excess of 70% by volume hydrogen is used (derived from AX or HNS gas) at temperatures ranging from about 600.degree. to about 750.degree. C.
While the above-mentioned U.S. Pat. No. 3,847,682 thus provides a method of strengthening low carbon steel strip or sheet, or articles formed therefrom by stamping or deep drawing, for optimum weldability and Charpy impact properties, the product requires denitriding after the alloy-nitrogen precipitation strengthening step. According to the prior art denitriding must be conducted in an atmosphere containing more than about 70% hydrogen at temperatures of about 700.degree. to about 750.degree. C., for periods of time ranging up to 40 hours. It is thus evident that such a denitriding step is expensive, commercially impractical and dangerous by reason of the high hydrogen content of the annealing atmosphere.
Application Ser. No. 607,624, filed Aug. 25, 1975, in the name of Rollin E. Hook, discloses a method of strengthening deep drawing quality low carbon steel sheet and strip stock, or article formed therefrom, containing from about 0.002 to about 0.015% carbon, up to about 0.012% nitrogen, up to about 0.08% aluminum, about 0.05 to about 0.6% manganese, up to about 0.035% sulfur, up to about 0.01% oxygen, up to about 0.01% phosphorus, up to about 0.015% silicon, a nitride-forming element chosen from the class consisting of titanium, columbium, zirconium, and mixtures thereof, in amounts such that titanium in solution is from about 0.02 to about 0.2%, columbium in solution is from about 0.025 to about 0.3%, and zirconium in solution is from about 0.025 to about 0.3%, the sum total of nitride-forming elements not exceeding about 0.3% in solution, and balance iron. The sheet and strip stock is annealed to produce complete recrystallization; the stock, or article formed therefrom, is then heat treated in an atmosphere comprising about 1 to about 20% by volume ammonia in a carrier gas of nitrogen and hydrogen at 593.degree. to 705.degree. C. (1100.degree. to 1300.degree. F.) for a period of time sufficient to cause reaction of the nitride-forming elements with ammonia to form small, uniformly dispersed nitrides and to form a surface layer of iron nitride, and denitrided in an atmosphere comprising about 6 to about 50% by volume hydrogen and balance nitrogen at 650.degree. to 760.degree. C. (1200.degree. to 1400.degree. F.) for a period of time sufficient to remove the iron nitride surface layer and to reduce nitrogen in solid solution to less than about 0.03% by weight. The yield strength is increased to at least about 50 ksi, and an average plastic strain ratio of at least about 1.8 is obtained.
While the strengthening methods disclosed in U.S. Pat. No. 3,847,682 and in application Ser. No. 607,624 achieve excellent properties in the final products, in most instances the heat treatment times extend beyond the range amenable to continuous annealing processes. Ordinarily the strengthening heat treatment for articles must be conducted as a batch type operation wherein the time required to heat the charge of strip, sheet or articles to the heat treatment temperature and the time required to cool the charge back to room temperature would appreciably exceed the time required at the temperature of heat treatment. The total heat treatment time or cycle time, as a measure of production rate, must include the heat-up and cool-down times as well as the time at temperature. Alternative strengthening methods which would substantially reduce the cycle time, would thus provide significant advances in economy of operation through higher production rates and a reduction in the number of heat treating units required to establish a given level of production.
Nitrocarburizing by means of molten cyanide and/or cyanate bath treatments is discussed in articles in "Heat Treatment of Metals", pp. 39-49 and 51-54 (1975). The article by T. Bell, entitled "Ferritic Nitrocarburising", at pp. 39-49 reviews molten salt bath treatments, gaseous nitrocarburizing and vacuum nitrocarburizing. It is indicated that all such treatments, when applied to ferritic steels, result in formation of an epsilon iron carbonitride phase on the surface which improves the tribological properties, fatigue resistance, and wear and anti-scuffing properties. It is stated that during ferritic nitrocarburizing only nitrogen diffuses inwardly from the carbonitride compound surface layer "because the ferrite is normally already at its equilibrium concentration with respect to carbon." With molten salt bath treatment, conducted at 570.degree. C. (1060.degree. F.) followed by atmosphere cooling, the depth of diffusion, as determined by nitrogen analysis, ranged from about 0.3 mm after 0.1 hour to about 2 mm after 10 hours treatment. A depth of about 1 mm was attained after 2 hours treatment. Gaseous nitrocarburizing obtained substantially identical results.